IB Physics/Electromagnetic waves

Light is an EM wave. The speed of light is independent of the velocity of the source of the light.

Electromagnetic waves consist of oscillating magnetic and electric fields that are at 90 degrees to each other and in phase. This can be visualised as two transverse waves perpendicular to each other, propagating in the same direction.

G.1.2 Describe the different regions of the electromagnetic spectrumEdit

The splitting of white light into its component colours when it passes from one medium to the other because different media have different refractive indices is called dispersion (as shown below). Red has the greatest wavelength therefore it has the lowest frequency, in comparison to blue light which has a short wavelength and a high frequency.

G.1.4 Describe the dispersion of EM waves in terms of the dependence of refractive index on wavelengthEdit

In the example above, red is diffracted the least, while violet the most. This is because the waves of lower frequency (i.e. red) are diffracted less (have a lower refractive index).

Dispersion can occur throughout the EM spectrum not just for light.

G.1.5 Distinguish between transmission, absorption and scattering of radiationEdit

Consider an EM wave going through a medium:

----------|medium|--------->

By common sense:

the wave is transmitted if it goes straight through

the wave is absorbed if the substances in the medium (e.g. molecules in air) absorb its energy. This energy can be re-emitted.

the wave is scattered if it bounces of particles in the medium into random directions.

Waves can be partially transmitted/absorbed/scattered.

G.1.6 Discuss examples of the transmission, absorption and scattering of EM radiationEdit

The most common example is; Why is the sky blue and sunsets red?

During the day: blue light is scattered by small dust particles in the atmosphere. If there was no atmosphere, the sky would be black.

During sunsets: the light must travel through more atmosphere. Blue light is scattered more than red light (because it has a shorter wavelength) so the blue light is scattered too much before it reaches you, therefore we only see the red light.

Other examples include; UV light being absorbed by the ozone layer and infra-red radiation being absorbed by greenhouse gases (causing global warming).

Monochromatic (mono=one, chromatic=colour) light is of a small range of wavelengths, for example from 500-501 nm. Light bulbs are NOT monochromatic as white light is a mixture of many different frequencies.

Coherent light means the waves are linked together, that is all the photons emitted are in phase with one another.

Laser stands for "Light Amplification by Stimulated Emission of Radiation".

Stimulated emission, refers to a process where 'electrons that are in an excited state' emit photons and drop to a lower state. This is caused by another photon passing by the excited electron (giving a total of two photons). By nature, both photons are of the same wavelength and are in phase (explanation is not required).

Lasers work by pumping lots of energy into the laser such that over 50% of the electrons in a medium are in an excited state. It then has two mirrors, one of which lets 1% of photons through (this is the light we see).

G.2.13 State the equation relating angular magnification to the focal lengths to lenses in an astronomical telescope in normal adjustmentEdit

G.2.14 Solve problems involving the compound and the astronomical telescopeEdit

G.2.15 Explain the meaning of spherical aberrations and of chromatic aberration as produced by a single lensEdit

You are expected to know of two types of optical aberrations, chromatic and spherical.

Chromatic aberrations are caused by the fact that different wavelengths of light are refracted different amounts by the glass in the lens which after all has a different density than air. This can be counteracted by using several lenses after each other that have different refractive indexes to effectively "bend the color back".

Spherical aberrations on the other hand are a result of that no lenses are perfectly spherical. Thus the light incident on the outer portions of the lens will be bent more than the light incident more in the center. To prevent this an aperture may be used to prevent light from hitting the outer portions of the lens.